1. Field of the Invention
The present invention relates to methods of measuring torque within a powertrain as well as to apparatus for carrying out such methods, and more particularly to such an assembly incorporating a sensor for measuring torque transmitted through the coupling plate.
2. Prior Art
Modern internal combustion engines for automobiles, utilise engine management systems which rely on dynamometer derived data for torque output as measured on a relatively small number of test engines. This approach cannot account for the variance in performance of volume produced engines, either across the range of production tolerances or over service life, and is therefore sub-optimal.
Engine torque output is dependent on a number of variables including: rpm, ignition advance, airflow, fuel flow, barometric pressure and ambient temperature. In a modern vehicle, some or all of these variables are measured continuously and used in conjunction with a multi-dimensional memory map stored in the engine control unit (ECU) in order to predict torque. Actual engine output torque is controlled by varying air flow, fuel flow and ignition advance in response to driver demand, i.e. the position of the accelerator pedal, and in accordance with ambient temperature and pressure conditions. Engine management maps may be determined in order to minimise fuel consumption and/or emissions or, for example in a racing car, to maximise torque and/or power.
In principle an internal combustion engine can be thought of as a “torque pump”. Within the discipline of control engineering, it is generally recognised that one of the most effective and accurate ways in which to control the output of a device is to directly measure the output variable of interest and use it as a real-time negative feedback signal in a closed loop control system. Since the principal output of an automotive internal combustion engine is torque, then a torque sensor placed as close as possible to the engine output, i.e. the rear end of the crankshaft, offers the ideal route to improved engine control.
A further benefit of accurate real time torque measurement is in the control of automatic transmissions since, if gear ratio changes are carried out at zero torque or other controlled torque values, improvements can be made in gear change smoothness or speed. The ability to measure torque directly at the engine output can lead to a significant step forward in this application.
Historically, direct measurement of torque in a powertrain has been primarily restricted to engine research and development using techniques such as:                a brake dynamometer to measure engine or engine+transmission torque outputs. This approach effectively averages the torque output over a time period dependent on the bandwidth of the instrumentation. However, because of rotary inertias distributed throughout the powertrain, this technique cannot yield the instantaneous torque output of the engine at the crankshaft output.        strain gauged torque cells are used routinely in R&D applications. However they are far too expensive for use in customer vehicles. They also require slip ring systems to transmit power and signals between the rotating powertrain and the stationary chassis or rig.        magneto-elastic torque sensors have been applied to test vehicles and some racing vehicles. They are applicable to shaft mounting and require typically 20-50 mm of shaft length for their installation. Such space is at a premium in production powertrains and may be unavailable in systems incorporating automatic transmissions. In addition magneto-elastic sensors are susceptible to stray magnetic fields which abound in production vehicles due to multiple electric motors and solenoids.        
In an automatic transmission equipped automotive powertrain, the engine output is transferred from a bolted flange at the end of the crankshaft, via a flexible steel disk (flexplate), to the torque converter by a second group of bolted fastenings (typically 3 or 4) on a significantly larger radius. The flexplate also carries the starter ring gear around its periphery.
Torque generated by the engine is transmitted by developing shear strain within the disk material, which in principal can be sensed by applying at least one pair of linear strain transducers oriented at +/−45′ to a line radiating from the centre of the flexplate. This approach to torque measurement in an essentially rigid disk component is understandable to those skilled in the art of transducer design and disclosed in DE 4208522. However, a flexible disk coupling component such as a flexplate, in its normal duty, is potentially subject to three forces and two couples apart from the desired couple (i.e. in-plane torque), and these extraneous loads can reduce the accuracy of the torque measured using a strain sensor mounted on the disk.
The problem of measuring torque in a flexplate (a standard automotive component which transfers torque from the crankshaft to the torque converter) is that the flexplate must be flexible (compliant) with regard to axial loading and to out-of-plane bending but stiff in torsion. The axial loading can be due to axial movement or expansion (due to internal pressure) of the torque converter, whereas the out-of-plane bending can be caused by any angular misalignment between the crankshaft axis and the automatic transmission axis. Flexplate compliance means that high stresses, due to axial loading and out-of-plane bending, which might lead to vibration within the vehicle and fatigue failure of the flexplate, are prevented. The successful design of a torque sensing flexplate hinges on the ability to be able to minimise and separate the unwanted strains due to axial loading and out-of-plane bending from the wanted strains due to engine torque. The prior art has not been able to achieve this goal.
There are numerous examples of sensors for measuring the torque or twist in a shaft with the purpose of monitoring its safe operation or to effect control of some upstream or downstream piece of equipment, for example in an engine or a driven wheel. Indeed it is recognised within the automotive industry, that sensing torque is theoretically one of the best ways for effecting control of internal combustion engines and for torque distribution within all-wheel-drive systems. However the reliable and cost effective provision of such torque sensors in automotive applications has thus far been problematic. For example, in an automotive crankshaft or gearbox output shaft, there is typically either no available physical space on the shaft in which to locate such a torque sensor or the strain field is inappropriate—either too low in strain or too high in strain gradient.
Prior art systems are known, such as that illustrated in FIG. 3 of the present application, in which torque in a shaft is monitored by measuring the strain, in particular the bending strain arising on the circumferential faces (at either end of the internal faces) of spokes through which the torque in the shaft is transmitted.